1. Field of the Invention
The present invention relates to a terminal structure of a coaxial cable connected to a terminal provided on a circuit board, and a connection structure of a terminal of this coaxial cable.
2. Description of the Related Art
Recently, coaxial cables have been used, for example, in information communication devices and medical devices to transmit communication signals. As shown in FIG. 3, a coaxial cable 110 of this type comprises an internal conductor 111, an internal insulating layer 113 which is provided on the outer peripheral surface of the internal conductor 111 and which covers the internal conductor 111, an external conductor 115 which is provided on the outer peripheral surface of the internal insulating layer 113 and which covers the internal insulating layer 113, and an external insulating layer 117 which is provided on the outer peripheral surface of the external conductor 115 and which covers the external conductor 115. The internal conductor 111 is a conductor which transfers communication signals. The internal insulating layer 113 is a dielectric. The external conductor 115 is a conductor which prevents the generation of, for example, electromagnetic noise. The external insulating layer 117 is a dielectric.
A terminal of the coaxial cable 110 is connected to terminals 123 and 125 provided on a circuit board 121. Therefore, in a structure (hereinafter, a terminal structure 131) of the terminal of the coaxial cable 110, the internal insulating layer 113, the external conductor 115, and the external insulating layer 117 are removed so that the internal conductor 111, the internal insulating layer 113, and the external conductor 115 are exposed in order from side of an end face 110b and the terminal is stepped. The internal conductor 111 is connected to one terminal 123 by unshown solder, and the external conductor 115 is connected to the other terminal 125 by unshown solder.
However, as shown in FIG. 3, the outside diameter of the internal conductor 111 is different from the outside diameter of the external conductor 115 covering the internal conductor 111 via the internal insulating layer 113. Thus, the internal conductor 111 and the external conductor 115 are not easily connected to the respective terminals 123 and 125 provided on the flat circuit board 121 because of the difference of their outer shapes. This may lead to unstable connection.
In response to the size reduction of the circuit board 121, a size reduction (space saving) has recently been required for a connection portion between the internal conductor 111 and terminal 123 and a connection portion between the external conductor 115 and terminal 125. Accordingly, the length of the terminal of the coaxial cable 110 including the exposed portions tends to be short. However, if the length of the terminal is short, a sufficient connection area may not be ensured.
Thus, for example, according to Jpn. Pat. Appln. KOKAI Publication No. 2008-257936, in order to compensate for the difference of outer shape between an internal conductor and an external conductor, a step corresponding to the difference is formed in a circuit board, and a terminal is provided to adapt to the step. This ensures stable connection and a sufficient connection area.
In the circuit board according to Jpn. Pat. Appln. KOKAI Publication No. 2008-257936 described above, the step is fabricated, and a space for the fabrication is needed. When the circuit board 121 is flat as shown in FIG. 3, the connection between the internal conductor 111 and terminal 123 and the connection between the external conductor 115 and terminal 125 are not stable, which may not ensure a connection area.